He helps to even up supply and exceed normal limits (4,3,8)
I believe the answer is:
push the envelope
'exceed normal limits' is the definition.
(I've seen this in another clue)
'he helps to even up supply' is the wordplay.
'supply' indicates an anagram (in a supple way).
'he'+'helps'+'to'+'even'+'up'='hehelpstoevenup'
'hehelpstoevenup' anagrammed gives 'PUSH THE ENVELOPE'.
'and' acts as a link.
(Other definitions for push the envelope that I've seen before include "Instruct postman at tight letter box" , "what postman does on reaching letterbox" , "Challenge" , "to try something new" , "achieve more than seems possible" .)